A measuring apparatus as used for measuring structures on wafers and masks used for their manufacture has been disclosed in the paper entitled “Pattern Placement Metrology for Mask Making” by Dr. Carola Bläsing. The paper was held at the Semicon Education Program Convention in Geneva on Mar. 31, 1998. The description given there is the basis for a coordinate measuring apparatus, of which various models are already commercially available.
An apparatus of the generic type is also disclosed, for example, in DE 199 49 005, DE 198 58 428, DE 101 06 699 or DE 10 2004 023 739. All the here cited documents of the state of the art, disclose a coordinate measuring apparatus for measuring structures or positions of structures on a substrate. Herein the substrate is placed on a stage moveable in the X coordinate direction and Y coordinate direction. The substrate to be measured is usually placed in a substrate support and exhibits a certain degree of bending. It is this degree of bending which must be calculated out, or the measured values must be corrected by a previously determined bending value for determination of the position or the structural width.
In order to calculate the bending of a mask or substrate, a great number of parameters first have to be known. The bending is dependent, for example, on the position of the support points of the substrate on the measuring stage or substrate support. A further influence on bending or on the calculation of bending originates from the mask size, the mask thickness, the position of the mask or the angular displacement of the mask in relation to the coordinate system of the measuring machine. For calculation of the degree of bending, it is assumed that the individual substrates are always placed on the measuring stage or on the substrate support in the same way. Additionally, the same values for mask size and mask thickness are always indicated, as, for example, those supplied by the manufacturer of the masks. Similarly, a constant (preset) modulus of elasticity is assumed for the calculation of the degree of bending. This modulus of elasticity is a material constant, which can also be supplied by the manufacturer of the mask. Deviations from the aforementioned values or parameters result in calculation of a degree of bending which does not correspond to the actual bending. Thus, for example, an error of 100 MPA in the modulus of elasticity results in an error of typically about one nanometer in the measured lateral structural position. This is, for example, the tolerance with which the mask is supplied. It is therefore of particular advantage, for example, to measure the modulus of elasticity of the currently inserted mask to enhance the accuracy of position determination of structures on a mask. Other parameters to be incorporated in the calculation of bending of the mask should also be determined individually for each mask, resulting in the enhancement of measuring accuracy.